1. Technical Field
This disclosure relates to optoelectronic devices, and in particular to high speed optoelectronic modulators for high data rate optical communications.
2. Description of the Related Art
For high speed optical communications, optoelectronic modulators are increasingly being used in a wide variety of applications ranging from satellite-based phased array radar to signal processing in communications systems. High-speed optoelectronic modulators are key components in many systems including optical communications, analog microwave optical links, and optical time division demultiplexing. In particular, the lithium niobate based modulator has received wide commercial acceptance for its efficient coupling to optical fibers and its low insertion loss. In some applications, III-V based modulators have been used as a substitute for lithium niobate based modulators since such III-V based modulators are well suited for integration with electronic and photonic devices, and may be fabricated to be more compact than their lithium niobate counterparts.
Typically, travelling wave III-V based modulators are fabricated in two configurations, namely, a) a microstrip configuration where a modulating electric field is applied perpendicular to the substrate plane, to bottom and top electrodes, and b) a coplanar configuration where a modulating electric field is applied between two electrodes that are placed side by side, separated by a gap.
Semiconductor travelling wave modulators generally require large drive voltages, since the implementation of relatively low driving voltages increases the device length. Increased length hampers the attainment of operating frequencies corresponding to millimeter wavelengths due to velocity mismatches caused by the microwave and optical refractive index difference.
Other optoelectronic modulators known in the art, such as GaAs-AlGaAs based modulators disclosed in R. Walker, "High-Speed III-V Semiconductor Intensity Modulators", IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 27, NO. 3, March 1991, pp. 654-667; may employ p-i-n structures that may exhibit relatively large optical propagation losses, may require driving voltages of 8 V or more, and may be relatively difficult to fabricate, such as by segmented etching or isolation.